Preparation and visible photocatalytic dye degradation of Mn-TiO₂/sepiolite photocatalysts

Abstract

The performance of Mn-TiO₂/sepiolite photocatalysts prepared by the sol-gel method and calcinated at different temperatures was studied in the photocatalytic degradation of direct fast emerald green dye under visible light irradiation, and a series of analytical techniques such as XRD, SEM, FTIR, TG-DSC, XPS, UV-vis-DRS and Raman spectroscopy were used to characterize the morphology, structure and optical properties of the photocatalysts. It is found that the anatase TiO₂ was formed in all photocatalysts. Mn⁴⁺ might incorporate into the lattice structure of TiO₂ and partially replace Ti⁴⁺, thus causing the defects in the crystal structure and the broadening of the spectral response range of TiO₂. At the same time, TiO₂ particles were dispersed on the surface of the sepiolite, which immobilized TiO₂ particles with sepiolite via the bond of Ti−O−Si. Mn-TiO₂/sepiolite calcined at 400 °C exhibits the highest photocatalytic activity and the degradation rate of direct fast emerald green is up to 98.13%. Meanwhile, it also shows good stability and universality.

Key words： Mn-TiO₂/sepiolite; calcination temperature; photocatalytic degradation; visible light irradiation; dye wastewater

Contents

Introduction

Experimental

Preparation of photocatalysts

Characterization of photocatalysts

Photocatalytic activity evaluation

Results and discussion

Characterization of Mn-TiO₂/sepiolite catalysts

The photocatalytic performance of Mn-TiO₂/sepiolite catalyst

Conclusions

Acknowledgements

References

Introduction

Dye effluents, abundant with various hazardous and noxious substances discharged by numerous dye-utilizing industries such as textiles, dyeing, paper and pulp, tannery and paint, and dye manufacturing, have a serious threat to human health and environment [¹–³]. Several methods have been performed to remove organic dyes from dye effluents such as adsorption [⁴], flocculation/coagulation [⁵‒⁶], membrane separation [⁷], biological processes [⁸] and semiconductor heterogeneous photocatalysis technology [⁹‒¹⁰]. Among them, the semiconductor heterogeneous photocatalysis has been regarded as one of the promising and cost-effective methods because of the lower energy consumable, milder condition, simple devices, easy operation and no secondary pollution [¹¹‒¹²]. As a well-known photocatalyst, TiO₂ has been broadly studied due to its intense oxidation ability, good stability, low cost and non-toxicity [¹³–¹⁵]. However, pure TiO₂ with a wide band-gap (E_g = 3.2 eV) can only utilize ultraviolet light and has high photogenerated electron‒hole pairs recombination probability [¹⁶]. Therefore, the efficient use of visible light (43% solar spectrum) and the suppression of photo-induced charges recombination become effective methods for developing TiO₂-based photocatalysts. The reformative methods have been proposed such as coupling of narrow band-gap semiconductors [¹⁷], aggradation of noble metals [¹⁸] and doping of metals or nonmetals [¹⁹], among which metal doping especially 3d transition-metal doping has been found to be an efficient way [²⁰‒²¹]. Mn can substitute Ti lattice sites as a dopant and favor the separation of electron‒hole pairs acting as a hole trap, which has the biggest potential among 3d transition-metals [²²] for the narrow band gap and the introduction of effective intermediate bands to allow multi-band optical absorption. Many researchers reported that the Mn-doped TiO₂ nanoparticles exhibit higher photocatalytic activity than bare TiO₂ under visible light irradiation [²³–²⁶]. Nevertheless, pure TiO₂ or doped TiO₂ are usually not easy to sedimentate, so it is difficult to recycle them after use which is still a prominent problem, thus limits its application. Recently, many studies proposed to immobilize doped TiO₂ with a carrier to enhance recycling capacity. In these studies, TiO₂ or doped TiO₂ is immobilized on a support such as zeolite [²⁷], activated carbon [²⁸], montmorillonite [²⁹], silica [³⁰], diatomite [³¹], sepiolite [³²] and other carriers. Among various carriers, sepiolite is regarded as a promising photocatalyst support due to its unique layered and reticulation of plant fiber structure, large surface area, high adsorption capacity, good chemical stability, low cost and abundance [³³–³⁵].

It is believed that using Mn as dopants and sepiolite to support Mn-doped TiO₂ can availably utilize the synergetic effect to obtain a series of Mn-TiO₂/sepiolite composites with superior photocatalytic performance. Generally, preparation conditions and especially the calcination temperature have a strong effect on the structure and the catalytic performance of photocatalysts [³⁶]. Although previous studies have investigated the influence of calcination temperature on the structure and performance of doped TiO₂ [³⁷] and sepiolite-supported TiO₂ separately [³⁸], no attention has been paid to sepiolite-supported Mn-doped TiO₂ to our knowledge.

In this work, Mn-TiO₂/sepiolite composite photocatalysts with different calcination temperatures were prepared by the sol-gel method and applied for the photocatalytic degradation of direct fast emerald green dye under visible light irradiation. The influence of calcination temperature on physicochemical and optical properties of Mn-TiO₂/sepiolite was investigated using XRD, SEM, FTIR, XPS, TG-DSC, UV-vis DRS and Raman spectroscopy. Besides, recycling studies were carried out to test recycling stability of such Mn-TiO₂/sepiolite photocatalysts.

Experimental

Preparation of photocatalysts

Mn-TiO₂/sepiolite composites, with 2 wt.% Mn ions and 50 wt.% TiO₂ and different calcination temperatures, were prepared by the sol-gel method at first. 6.27 mL tetrabutyl titanate was added into the 48 mL absolute ethanol with a constant stirring. Then 1.5 g sepiolite was gradually added into the solution with continuous vigorous stirring for 0.5 h. The suspension was labeled as Solution A. The doped solution was obtained by mixing 18.825 g Mn(NO₃)₂·4H₂O with 10 mL absolute ethanol and a small amount of deionized water. After that, the pH of the solution was adjusted to 2 with nitric acid. The sol solution was assigned as Solution B. Solution B was then added dropwise into Solution A with vigorous stirring, and the continuous stirring maintained for a few minutes until gel formed. After aging for 12 h and drying at 80 °C for about 6 h, the products were calcined at different temperatures of 300, 400, 500 and 600 °C for 2 h to obtain Mn-TiO₂/sepiolite composites.

Characterization of photocatalysts

XRD patterns were obtained using PANalytical B.V. with Cu Kα radiation (l = 1.54060 Å) from 5° to 60°. The microstructure of the samples was investigated by SEM. FTIR spectra were obtained using WQF-520 spectrophotometer in the range of 400‒4000 cm⁻¹. The simultaneous TG-DSC system (NETZSCH STA449C) was applied for the thermal property analysis at the temperature range from 40 to 1000 °C. XPS measurements were performed on a Thermo ESCALAB 250Xi spectrometer using Al Kα radiation (1486.6 eV and 12.5 kV) and all the binding energies were calibrated by adjusting the C 1s peak to 284.6 eV. UV-vis-DRS spectra were recorded by a UV-vis spectrophotometer (PerkinElmer Lambda 850) in the scanning range of 190‒800 nm using BaSO₄ as standard material. Raman spectroscopy was measured on an ID Raman microIM-52 spectrometer (Ocean Optic) using a laser with 785 nm excitation wavelength.

Photocatalytic activity evaluation

The photocatalytic activity of Mn-TiO₂/sepiolite was evaluated by the degradation of direct fast emerald green dye aqueous solution (50 mg·L⁻¹) in a BL-GHX-V photoreactor at room temperature. All initial pH of the direct fast emerald green dye solution was adjusted to 3.0 for each experiment. 25 mg photocatalyst was dispersed in 50 mL dye solution. The mixture was stirred in the dark for 30 min to establish an adsorption‒desorption equilibrium between the direct fast emerald green dye and the photocatalyst before irradiation. Then the mixture was exposed to a 23 W visible lamp under magnetic stirring for 120 min. At defined time intervals, 3 mL of solution sample was extracted and centrifuged, the concentration of direct fast emerald green dye was obtained by measuring the absorbance at the maximum absorption wavelength (l = 622 nm) using a UV-visible spectrophotometer (UV-1800, Shimadzu). The observed absorbance was calculated to concentration by applying the Lambert‒Beer’s law, and the degradation rate h can be calculated by the following equation:

\eta /\%={\displaystyle \frac{c_{\mathrm{0}}-c}{c_{\mathrm{0}}}}\times \mathrm{100}={\displaystyle \frac{A_{\mathrm{0}}-A}{A_{\mathrm{0}}}}\times \mathrm{100}

where c₀ and c are concentrations of the direct fast emerald green dye before and after the degradation, respectively; A₀ is the initial absorbance of direct fast emerald green dye solution; and A is the final absorbance. All the experiments were performed under identical conditions involving visible light irradiation, pH, and photocatalyst dosage.

Results and discussion

Characterization of Mn-TiO₂/sepiolite catalysts

The XRD patterns of Mn-TiO₂/sepiolite catalyst calcined at different temperatures are shown in Fig. 1. It can be seen that the distinct sepiolite and anatase TiO₂ are observed in all catalysts [³⁸‒³⁹]. And the characteristic peaks of anatase TiO₂ become sharper and more intense with the calcination temperature increasing, which indicates an improvement in crystallinity of anatase and the size of the crystalline particles. When the calcination temperature reached 600 °C, the diffraction peaks correspond to rutile TiO₂ appeared [³⁸,⁴⁰], which indicates that part of anatase TiO₂ is transformed to rutile type. It can also be found that the characteristic peaks of sepiolite shift to a higher diffraction angle (9.73° and 10.74°) in the Mn-TiO₂/sepiolite calcined at 600 °C, indicating that the interlayer space of sepiolite is decreased, which may attribute to the removal of interlaminar structure water of the sepiolite. Besides, there are not any other crystalline phases of Mn observed in any of the samples. However, it should not be neglected that the peaks of TiO₂ shifts of small magnitude in all samples, indicating that Mn has substituted part of Ti sites in the TiO₂ lattice.

Fig.1 XRD patterns of Mn-TiO₂/sepiolite prepared at different calcination temperatures: 600 °C (a); 500 °C (b); 400 °C (c); 300 °C (d).

Full size|PPT slide

Figure 2 presents FTIR spectra of pure sepiolite and Mn-TiO₂/sepiolite catalysts prepared under different calcination temperatures. As shown in Fig. 2, the absorption peaks around 3390 and 1629 cm⁻¹ can be ascribed to the stretching vibration and bending vibration of surface hydroxyl groups and physically absorbed water on the surface of oxides [⁴¹]. The intensity of peaks around 3414 and 1651 cm⁻¹ decreased significantly when the calcination temperature is more than 500 °C, indicating that the content of hydroxyl in samples is greatly reduced, which may lead to the decrease of the adsorption performance of the sample [³⁹]. The absorption peaks at around 1152, 948, 750 and 673 cm⁻¹ correspond to the symmetric and the antisymmetric stretching vibrations, the bending vibrations inside and outside the surface of Si−O−Si in the sepiolite, respectively. Compared with pure sepiolite, the absorption band of Si−O−Si at around 1150 cm⁻¹ gets narrowed and shifts to the lower wavenumber, and a new Ti−O−Si stretching vibration absorption appears at 1161 cm⁻¹ in all Mn-TiO₂/sepiolite samples, resulting in a growth of the Si−O−Si bond as well as implying the composited TiO₂ and sepiolite by the Ti−O−Si chemical combination, which can be attributed to the fact that TiO₂ particles were dispersed on the surface of sepiolite and Ti−O−Si immobilized TiO₂ particles with sepiolite. Moreover, there is no absorption peak of the Mn−O bonding detected for all catalysts, which may be attributed to the incorporation of Mn ions into interstitial sites of the TiO₂ lattice, consistent with the XRD result.

Fig.2 FTIR spectra of pure sepiolite and Mn-TiO₂/sepiolite prepared at different calcination temperatures: Mn-TiO₂/sepiolite (600 °C) (a); MnTiO₂/sepiolite (500 °C) (b); Mn-TiO₂/sepiolite (400 °C) (c); Mn-TiO₂/sepiolite (300 °C) (d); sepiolite (e).

Full size|PPT slide

TG-DSC curves of the Mn-TiO₂/sepiolite precursor are presented in Fig. 3, which involves four main stages of weight loss. The first step of weight loss (12.73%) in the range of 40‒150 °C, corresponding to an endothermal peak (103 °C) in the DSC curve, can be ascribed to the evaporation of the surface absorbed water in the MnTiO₂/sepiolite precursor [⁴²]. The second weight loss (6.34%) from 150 to 230 °C accompanied by two exothermic peaks (128 and 224 °C) is attributed to the decomposition of manganese compounds and butyl titanate [⁴³]. The third stage weight loss of 3.91% in the range of 230‒500 °C corresponds to an exothermic peak at 300 °C, which is due to the phase transition from amorphous TiO₂ to crystalline anatase. The last step of weight loss of 3.21% in the range of 500‒1000 °C corresponding to two exothermic peaks (542, 669 and 870 °C) in the DSC curve can be ascribed to the phase transition of TiO₂ from anatase to rutile and the depth removal of water in the interlaminar structure of sepiolite, respectively. In view of rutile phase of TiO₂ has a lower photocatalytic efficiency compared to anatase phase [³⁸,⁴⁴], and the depth removal of interlaminar structure water will lead to the decrease of the surface area and destruction of the sepiolite structure. In general, the optimum heat treatment temperature of the Mn-TiO₂/sepiolite should be 300‒500 °C.

Fig.3 TG-DSC curves of as-prepared Mn-TiO₂/sepiolite precursor.

Full size|PPT slide

The morphology of sepiolite and Mn-TiO₂/sepiolite (400 °C) sample is studied by using SEM displayed in Fig. 4. It can be seen that sepiolite exhibited obvious fibrous morphology (Fig. 4(a)). From Fig. 4(b), many TiO₂ particles were uniformly dispersed on the surface of the composites, and TiO₂ particles are substantially bonded with sepiolite. The fibrous morphology of sepiolite was essentially preserved in the calcination process at all preparation times, which is in agreement with XRD analysis.

Fig.4 SEM images of samples: (a) sepiolite; (b) Mn-TiO₂/sepiolite (400 °C).

Full size|PPT slide

Figure 5 shows XPS spectra of Mn 2p, Ti 2p, and O 1s for Mn-TiO₂/sepiolite calcined at 400 °C. In Fig. 5(a), two well-defined peaks located at 653.1 and 641.4 eV correspond to Mn 2p_1/2 and Mn 2p_3/2 binding energies, respectively, implying that Mn is in the form of Mn⁴⁺ oxidation state in the catalyst [⁴³]. Figure 5(b) displays two typical peaks at 463.8 and 458.1 eV belonging to Ti 2p_1/2 and Ti 2p_3/2, respectively, which indicates that the valence of Ti ions is+4 in the Mn-TiO₂/sepiolite [⁴⁵‒⁴⁶]. Figure 5(c) shows O 1s spectra of Mn-TiO₂/sepiolite, which is distinct that the main O 1s peaks show two features peaks. The main peaks located at 529.6 eV correspond to lattice oxygen O²⁻ bound to Ti and/or Mn, whereas the peaks at 532.2 eV are attributed to the surface chemisorbed oxygen or defect oxide such as O²⁻ or SiO₂ species [⁴³,⁴⁵].

Fig.5 XPS spectra of Mn-TiO₂/sepiolite: (a) Mn 2p; (b) Ti 2p; (c) O 1s.

Full size|PPT slide

UV-vis-DRS spectra of pure TiO₂ and Mn-TiO₂/sepiolite catalysts calcinated at different temperatures are shown in Fig. 6. For pure TiO₂, it shows optical absorption capacity only in the UV region, while Mn-TiO₂/sepiolite composites present strong absorption capacity not only in the UV region, but also in the visible region. It is probably attributed to the doping of Mn⁴⁺ on the TiO₂ particles during the calcination, because the ionic radii of Mn⁴⁺ is close to that of Ti⁴⁺. The incorporation of Mn⁴⁺ leads to the formation of impurity levels, which results in a narrower band gap for Mn-TiO₂/sepiolite composites [²²‒²³]. Meanwhile, the red-shift of the absorption edge is improved with calcination temperature increasing. In general, this shift is attributed to the narrowing of the band gap energy, which leads to lower energy required for the excitation of electrons from valence band (VB) to conduction band (CB) [³⁷], and the order of the visible light absorption ability of the samples is A_{400 °C}>A_{600 °C}>A_{500 °C}>A_{300 °C}.

Fig.6 UV-vis-DRS spectra of pure TiO₂ and Mn-TiO₂/sepiolite prepared with different calcination temperatures: Mn-TiO₂/sepiolite (600 °C) (a); Mn-TiO₂/sepiolite (500 °C) (b); Mn-TiO₂/sepiolite (400 °C) (c); Mn-TiO₂/sepiolite (300 °C) (d); TiO₂ (e).

Full size|PPT slide

Figure 7 presents Raman spectra of the Mn-TiO₂/sepiolite catalyst prepared under different calcination temperatures. One scattering peak appears in the spectrum at 283 cm⁻¹ corresponding to SiO₂ of sepiolite. The observed peaks at 395 cm⁻¹ are ascribed to the B_1g peak of TiO₂ [⁴⁷]. The spectra exhibit three peaks at 411, 515 and 635 cm⁻¹ which are characteristic of the anatase phase of TiO₂ [¹⁴,⁴⁶], and the intensity of 411, 515 and 635 cm⁻¹ bands of Mn-TiO₂/sepiolite samples builds up with calcination temperatures changing in the turn of 600, 300, 500 and 400 °C, indicating that the amount of anatase TiO₂ in Mn-TiO₂/sepiolite samples increases. In general, the amount of anatase TiO₂ correlates well with the photocatalytic activity of TiO₂-based photocatalysts [⁴⁴]. Moreover, from Fig. 6, it also can be seen that a characteristic scattering peak of rutile TiO₂ with appreciable intensity appears at 438 cm⁻¹ in the Mn-TiO₂/sepiolite catalyst calcined at 600 °C [⁴⁸], which indicates the part of anatase TiO₂ phase transition to rutile, and it is consistent with the results of XRD and TG-DSC.

Fig.7 Raman spectra of Mn-TiO₂/sepiolite prepared at different calcination temperatures: 600 °C (a); 500 °C (b); 400 °C (c); 300 °C (d).

Full size|PPT slide

The photocatalytic performance of Mn-TiO₂/sepiolite catalyst

Figure 8 shows the photocatalytic performance of the Mn-TiO₂/sepiolite catalyst prepared under different calcination temperatures for direct fast emerald green dye degradation under visible light irradiation. From Fig. 8, it can be seen that the calcination temperature has a significant influence on the photocatalytic performance of Mn-TiO₂/sepiolite catalysts, and the catalyst calcined at 400 °C exhibits the highest photocatalytic activity with a degradation rate of 98.13%. The order of the degradation rate of Mn-TiO₂/sepiolite catalysts is h_{400 °C}>h_{500 °C}>h_{300 °C}>h_{600 °C}. Combined with the characterization results, the order of the visible light absorption ability of samples is A_{400 °C}>A_{600 °C}>A_{500 °C}>A_{300 °C}. Generally, strong capacity of visible light absorption over photocatalyst can be favorable to visible-light catalytic activity. However, the catalytic activity of the photocatalyst is not only depended on the optical absorptivity, but also rest with other factors [⁴⁹]. Titanium species in the sample calcined at 300 °C are not completely transformed into anatase TiO₂ that leads to a relatively low proportion of TiO₂ active ingredient and low photocatalytic activity. At the same time, titanium species in the sample calcined at 600 °C transformed not only into anatase TiO₂ but also into low active rutile TiO₂, resulting in the catalytic deactivation. Besides, both the decreased interlayer spacing of sepiolite and the largest crystal size in the catalyst calcined at 600 °C are reasons leading to the decline of the catalytic activity. The above results indicate that an appropriate calcination temperature is advantageous to enhance the photoactivity of Mn-TiO₂/sepiolite for the degradation of direct fast emerald green dye under visible light irradiation. Consequently, the optimum calcination temperature of Mn-TiO₂/sepiolite is 400 °C.

Fig.8 Photocatalytic activity of Mn-TiO₂/sepiolite calcined at different temperatures: dye stock solution (a); Mn-TiO₂/sepiolite (600 °C) (b); Mn-TiO₂/sepiolite (300 °C) (c); Mn-TiO₂/sepiolite (500 °C) (d); Mn-TiO₂/sepiolite (400 °C) (e).

Full size|PPT slide

Hydroxyl radicals (∙OH), superoxide radical anions (·O₂⁻) and photogenerated holes (h⁺) species all have been proven to be main causes for the photocatalytic degradation reaction [⁴⁰,⁵⁰‒⁵¹]. In this study, to investigate the major reactive species during the photocatalytic degradation of direct fast emerald green dye by Mn-TiO₂/sepiolite composite calcined at 400 °C, the trapping experiments using isopropanol (IPA), K₂C₂O₄ and EDTA-2Na as scavengers for ·OH, ·O₂⁻ and h⁺ are performed during the photocatalytic process individually. As shown in Fig. 9, the degradation rates of direct fast emerald green dye under 120 min of visible light irradiation decline from 98.13% to 83.78%, 70.08% and 66.72% after the addition of EDTA-2Na, K₂C₂O₄ and IPA, respectively. Based on these results, ∙O₂⁻ and h⁺ are the main active species responsible for the photocatalytic degradation of contaminants, and ·OH plays a subordinate role in the direct fast emerald green dye degradation.

Fig.9 Photocatalytic activity of the Mn-TiO₂/sepiolite composite for the degradation of direct fast emerald green dye in the presence of different scavengers.

Full size|PPT slide

Based on the aforementioned experimental results, a schematic photocatalytic mechanism of the Mn-TiO₂/sepiolite nanocomposite system for the degradation of dye is proposed (Fig. 10). When the calcination temperature reaches 400 °C, anatase TiO₂ is formed in the photocatalysts. Mn⁴⁺ might incorporate into the lattice structure of TiO₂ and partially replace Ti⁴⁺, thus causing the defects in the crystal structure and the broadening of the spectral response range of TiO₂. The sepiolite plays the main role for adsorption and fixed load, and the larger surface of photocatalysts provided more surface active sites for adsorption and photocatalytic reaction. Because of electrostatic attraction between the positively charged hole on TiO₂ and the negatively charged sepiolite, the excited electrons and holes on TiO₂ migrated efficiently holes and thus suppressed the charge recombination [⁵²]. After absorbing dyes on the surface of sepiolite, TiO₂ nanoparticles are excited to produce photogenerated electrons and holes [⁵³]. Then, e⁻ in TiO₂ is scavenged by O₂ on the surface of the photocatalyst to generate ∙O₂⁻ radicals. h⁺ in the VB capture and further react with water to form active ·OH. The photocatalytic degradation reaction would subsequently proceed, and these ·O₂⁻ radicals and h⁺ can directly oxidize organic dyes into water, carbon dioxide and mineral salts.

Fig.10 The schematic illustration for the possible photocatalytic mechanism of Mn-TiO₂/sepiolite samples.

Full size|PPT slide

The stability of Mn-TiO₂/sepiolite calcined at 400 °C for direct fast emerald green dye photodegradation under visible light was studied under the same conditions. At the end of each cycle, the Mn-TiO₂/sepiolite catalyst sediment was centrifuged and then the catalyst was washed several times with distilled water, and dried, and then calcined at 400 °C. As shown in Fig. 11, the obtained Mn-TiO₂/sepiolite maintained a high photocatalytic activity after five consecutive recycles. Thus, it is demonstrated that Mn-TiO₂/sepiolite has good stability.

Fig.11 Cycling runs of Mn-TiO₂/sepiolite calcined at 400 °C for the photodegradation of direct fast emerald green dye.

Full size|PPT slide

Another reason is that dye wastewater is difficult to remove because of its complex ingredients. Thus, four other dyes, rosewood, acid red, apricot yellow and coffee brown, with the initial concentration of 50 mg∙L⁻¹, are chosen to evaluate the universality of the Mn-TiO₂/sepiolite calcined at 400 °C for photodegradation of organic dyes under visible light irradiation. The results are shown in Fig. 12. It can be seen from Fig. 12 that the Mn-TiO₂/sepiolite sample has good degradation performance for these four dyes, and the degradation rate can reach more than 85% in 120 min, which indicates that the Mn-TiO₂/sepiolite catalyst has relatively wide applications in the dyes degradation under visible light.

Fig.12 The photocatalytic efficiency of the Mn-TiO₂/sepiolite composite for different dyes.

Full size|PPT slide

Conclusions

The performance of Mn-TiO₂/sepiolite photocatalysts prepared by the sol-gel method and calcinated at different temperatures are studied in the photocatalytic degradation of direct fast emerald green dye under visible light irradiation. The physicochemical properties of Mn-TiO₂/sepiolite photocatalysts were characterized by XRD, SEM, FTIR, TG-DSC, XPS, UV-vis-DRS and Raman spectroscopy. The results show that anatase TiO₂ are in all photocatalysts, and TiO₂ particles were dispersed on the surface of sepiolite via the bond of Ti−O−Si. The doping of Mn⁴⁺ has incorporated into the lattice of TiO₂ and broadens the light absorption range of TiO₂ to the visible region. Mn-TiO₂/sepiolite calcined at 400 °C exhibits the highest catalytic activity with a degradation rate of direct fast emerald green dye up to 98.13% under visible light irradiation in 120 min, and the order of the photocatalytic activity (x) of Mn-TiO₂/sepiolite is x_{400 °C}>x_{500 °C}>x_{300 °C}>x_{600 °C}. This can be attributed to the higher ratio of anatase TiO₂ and the stronger visible light absorption capacity for the Mn-TiO₂/sepiolite composite calcined at 400 °C. In addition, Mn-TiO₂/sepiolite calcined at 400 °C has excellent degradation performance for other four dyes, and exhibits good stability after five cycles of usage. Both h⁺ and ∙O₂⁻ are found to contribute as the main active species in the trapping experiment for the decomposition of dye under visible light irradiation. The experimental results are helpful to solve dye pollution in wastewater and provide a modification idea for future research.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21406184) and the Foundation of Youth Science and Technology Innovation Team of Sichuan Province (Grant No. 2015TD0007).

References

1 Liang J, Ning X A, Sun J, . Toxicity evaluation of textile dyeing effluent and its possible relationship with chemical oxygen demand. Ecotoxicology and Environmental Safety, 2018, 166: 56–62

PMID

2 Xue F, Tang B, Bin L, . Residual micro organic pollutants and their biotoxicity of the effluent from the typical textile wastewater treatment plants at Pearl River Delta. Science of the Total Environment, 2019, 657: 696–703

PMID

3 Ozturk E, Karaboyacı M, Yetis U, . Evaluation of Integrated Pollution Prevention Control in a textile fiber production and dyeing mill. Journal of Cleaner Production, 2015, 88: 116–124

4 Liu J, Wang N, Zhang H, . Adsorption of Congo red dye on Fe_xCo_3−xO₄ nanoparticles. Journal of Environmental Management, 2019, 238: 473–483

PMID

5 Liu M, Zhu P, Yang W, . Synthesis and evaluation of a novel inorganic–organic composite flocculant, consisting of chitosan and poly-ferric cerium silicate. Journal of Chemical Technology and Biotechnology, 2019, 94(1): 79–87

6 Liu M, Zhu P, Ran Y, . Preparation, characterization and coagulation performance of a composite coagulant formed by the combination of polyferric sulfate (PFS) and Ce³⁺. Desalination and Water Treatment, 2016, 57(29): 13600–13607

7 Liu H, Chen Y, Zhang K, . Poly (vinylidene fluoride) hollow fiber membrane for high-efficiency separation of dyes-salts. Journal of Membrane Science, 2019, 578: 43–52

8 Karim M E, Dhar K, Hossain M T. Decolorization of textile reactive dyes by bacterial monoculture and consortium screened from textile dyeing effluent. Journal of Genetic Engineering & Biotechnology, 2018, 16(2): 375–380

PMID

9 Zhu P, Chen Y, Duan M, . Structure and properties of Ag₃PO₄/diatomite photocatalysts for the degradation of organic dyes under visible light irradiation. Powder Technology, 2018, 336: 230–239

10 Nguyen C, Fu C, Juang R. Degradation of methylene blue and methyl orange by palladium-doped TiO₂ photocatalysis for water reuse: Efficiency and degradation pathways. Journal of Cleaner Production, 2018, 202: 413–427

11 Aarthi T, Narahari P, Madras G. Photocatalytic degradation of Azure and Sudan dyes using nano TiO₂. Journal of Hazardous Materials, 2007, 149(3): 725–734

PMID

12 Chen Y, Zhu P, Duan M, . Fabrication of a magnetically separable and dual Z-scheme PANI/Ag₃PO₄/NiFe₂O₄ composite with enhanced visible-light photocatalytic activity for organic pollutant elimination. Applied Surface Science, 2019, 486: 198–211

13 Asadi A, Akbarzadeh R, Eslami A, . Effect of synthesis method on NS-TiO₂ photocatalytic performance. Energy Procedia, 2019, 158: 4542–4547

14 Ling L, Feng Y, Li H, . Microwave induced surface enhanced pollutant adsorption and photocatalytic degradation on Ag/TiO₂. Applied Surface Science, 2019, 483: 772–778

15 Dubnová L, Zvolská M, Edelmannová M, . Photocatalytic decomposition of methanol-water solution over N-La/TiO₂ photocatalysts. Applied Surface Science, 2019, 469: 879–886

16 Liu S, Qi K, Qiu M. Photocatalytic performance of TiO₂ nanocrystals with/without oxygen defects. Chinese Journal of Catalysis, 2018, 39(4): 867–875 (in Chinese)

17 Qin R, Meng F, Khan M, . Fabrication and enhanced photocatalytic property of TiO₂‒ZnO composite photocatalysts. Materials Letters, 2019, 240: 84–87

18 Zhang J, Huang Y, Dan Y, . P3HT/Ag/TiO₂ ternary photocatalyst with significantly enhanced activity under both visible light and ultraviolet irradiation. Applied Surface Science, 2019, 488: 228–236

19 Chen Q, Zhang Y, Zhang D, . Ag and N co-doped TiO₂ nanostructured photocatalyst for printing and dyeing wastewater. Journal of Water Process Engineering, 2017, 16: 14–20

20 Bashiri R, Mohamed N, Kait C, . Effect of preparation parameters on optical properties of Cu and Ni doped TiO₂ photocatalyst. Procedia Engineering, 2016, 148: 151–157

21 Anwar D, Mulyadi D. Synthesis of Fe-TiO₂ composite as a photocatalyst for degradation of methylene blue. Procedia Chemistry, 2015, 17: 49–54

22 Sharotri N, Sud D. Visible light responsive Mn‒S-co-doped TiO₂ photocatalyst — Synthesis, characterization and mechanistic aspect of photocatalytic degradation. Separation and Purification Technology, 2017, 183: 382–391

23 Binas V, Stefanopoulos V, Kiriakidis G, . Photocatalytic oxidation of gaseous benzene, toluene and xylene under UV and visible irradiation over Mn-doped TiO₂ nanoparticles. Journal of Materiomics, 2019, 5(1): 56–65

24 Quan F, Hu Y, Zhang X, . Simple preparation of Mn‒N-codoped TiO₂ photocatalyst and the enhanced photocatalytic activity under visible light irradiation. Applied Surface Science, 2014, 320: 120–127

25 Tbessi I, Benito M, Molins E, . Effect of Ce and Mn co-doping on photocatalytic performance of sol-gel TiO₂. Solid State Sciences, 2019, 88: 20–28

26 Divya Lakshmi K V, Siva Rao T, Swathi Padmaja J, . Visible light driven mesoporous Mn and S co-doped TiO₂ nano material: Characterization and applications in photocatalytic degradation of indigocarmine dye and antibacterial activity. Environmental Nanotechnology, Monitoring & Management, 2018, 10: 494–504

27 Wang Y, Chen J, Lei X, . Preparation of microporous zeolites TiO₂/SSZ-13 composite photocatalyst and its photocatalytic reactivity. Microporous and Mesoporous Materials, 2017, 250: 9–17

28 Sun S, Zhao R, Xie Y, . Photocatalytic degradation of aflatoxin B1 by activated carbon supported TiO₂ catalyst. Food Control, 2019, 100: 183–188

29 Bahrudin N, Nawi M. Mechanistic of photocatalytic decolorization and mineralization of methyl orange dye by immobilized TiO₂/chitosan-montmorillonite. Journal of Water Process Engineering, 2019, 31: 100843

30 Ferreira-Neto E, Ullah S, Simões M, . Solvent-controlled deposition of titania on silica spheres for the preparation of SiO₂@TiO₂ core@shell nanoparticles with enhanced photocatalytic activity. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2019, 570: 293–305

31 Zhang G, Sun Z, Duan Y, . Synthesis of nano-TiO₂/diatomite composite and its photocatalytic degradation of gaseous formaldehyde. Applied Surface Science, 2017, 412: 105–112

32 Hu X, Sun Z, Song J, . Synthesis of novel ternary heterogeneous BiOCl/TiO₂/sepiolite composite with enhanced visible-light-induced photocatalytic activity towards tetracycline. Journal of Colloid and Interface Science, 2019, 533: 238–250

PMID

33 Papoulis D, Panagiotaras D, Tsigrou P, . Halloysite and sepiolite‒TiO₂ nanocomposites: Synthesis characterization and photocatalytic activity in three aquatic wastes. Materials Science in Semiconductor Processing, 2018, 85: 1–8

34 Liu R, Ji Z, Wang J, . Solvothermal synthesized Ag-decorated TiO₂/sepiolite composite with enhanced UV-vis and visible light photocatalytic activity. Microporous and Mesoporous Materials, 2018, 266: 268–275

35 Zhou F, Yan C, Liang T, . Photocatalytic degradation of Orange G using sepiolite‒TiO₂ nanocomposites: Optimization of physicochemical parameters and kinetics studies. Chemical Engineering Science, 2018, 183: 231–239

36 Wang Q, Shi X, Xu J, . Highly enhanced photocatalytic reduction of Cr(VI) on AgI/TiO₂ under visible light irradiation: Influence of calcination temperature. Journal of Hazardous Materials, 2016, 307: 213–220

PMID

37 Cheng Y, Luo F, Jiang Y, . The effect of calcination temperature on the structure and activity of TiO₂/SiO₂ composite catalysts derived from titanium sulfate and fly ash acid sludge. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018, 554: 81–85

38 Zhou F, Yan C, Wang H, . Sepiolite‒TiO₂ nanocomposites for photocatalysis: Synthesis by microwave hydrothermal treatment versus calcination. Applied Clay Science, 2017, 146: 246–253

39 Hoseini S, Pirzaman A, Aroon M, . Photocatalytic degradation of 2,4-dichlorophenol by Co-doped TiO₂ (Co/TiO₂) nanoparticles and Co/TiO₂ containing mixed matrix membranes. Journal of Water Process Engineering, 2017, 17: 124–134

40 Wang W, Chen H, Fang J, . Large-scale preparation of rice-husk-derived mesoporous SiO₂@TiO₂ as efficient and promising photocatalysts for organic contaminants degradation. Applied Surface Science, 2019, 467‒468: 1187–1194

41 Sánchez-Rodríguez D, Méndez Medrano M G, Remita H, . Photocatalytic properties of BiOCl‒TiO₂ composites for phenol photodegradation. Journal of Environmental Chemical Engineering, 2018, 6(2): 1601–1612

42 Li X, Qian J, Xu J, . Synthesis and electrical properties of antimony-doped tin oxide-coated TiO₂ by polymeric precursor method. Materials Science in Semiconductor Processing, 2019, 98: 70–76

43 Zhang A, Zhang Z, Chen J, . Effect of calcination temperature on the activity and structure of MnO_x/TiO₂ adsorbent for Hg⁰ removal. Fuel Processing Technology, 2015, 135: 25–33

44 Bojinova A, Kralchevska R, Poulios I, . Anatase/rutile TiO₂ composites: Influence of the mixing ratio on the photocatalytic degradation of Malachite Green and Orange II in slurry. Materials Chemistry and Physics, 2007, 106(2‒3): 187–192

45 Karami A, Salehi V. The influence of chromium substitution on an iron-titanium catalyst used in the selective catalytic reduction of NO. Journal of Catalysis, 2012, 292: 32–43

46 Gao Q, Si F, Zhang S, . Hydrogenated F-doped TiO₂ for photocatalytic hydrogen evolution and pollutant degradation. International Journal of Hydrogen Energy, 2019, 44(16): 8011–8019

47 Pérez-Larios A, Hernández-Gordillo A, Morales-Mendoza G, . Enhancing the H₂ evolution from water-methanol solution using Mn²⁺–Mn³⁺–Mn⁴⁺ redox species of Mn-doped TiO₂ sol-gel photocatalysts. Catalysis Today, 2016, 266: 9–16

48 Aljawfi R, Vij A, Chae K, . Effects of rapid thermal annealing on the local environment, electronic structure and magnetic properties of Mn doped TiO₂ thin films. Applied Surface Science, 2018, 445: 287–297

49 Zhu P, Chen Y, Duan M, . Enhanced visible photocatalytic activity of Fe‒Cu‒ZnO/graphene oxide photocatalysts for the degradation of organic dyes. Canadian Journal of Chemical Engineering, 2018, 96(7): 1479–1488

50 Chachvalvutikul A, Jakmunee J, Thongtem S, . Novel FeVO₄/Bi₇O₉I₃ nanocomposite with enhanced photocatalytic dye degradation and photoelectrochemical properties. Applied Surface Science, 2019, 475: 175–184

51 Guo R, Zhu G, Gao Y, . Synthesis of 3D Bi₂S₃/TiO₂ NTAs photocatalytic system and its high visible light driven photocatalytic performance for organic compound degradation. Separation and Purification Technology, 2019, 226: 315–322

52 Ma J, Liu Q, Zhu L, . Visible light photocatalytic activity enhancement of Ag₃PO₄ dispersed on exfoliated bentonite for degradation of rhodamine B. Applied Catalysis B: Environmental, 2016, 182: 26–32

53 Sun Q, Li H, Niu B, . Nano-TiO₂ immobilized on diatomite: characterization and photocatalytic reactivity for Cu²⁺ removal from aqueous solution. Procedia Engineering, 2015, 102: 1935–1943